1. Field of the Invention
This invention relates to a method of enhancing cognitive function in a patient who has diminished cognitive function due to ADDL neurotoxicity by administering a non-peptidic compound having a molecular weight of less than 1000.
2. State of the Art
Alzheimer's disease (AD) is a fatal progressive dementia that has no cure at present. Although the molecular basis of the disease is not established, considerable evidence now implicates neurotoxins derived from amyloid beta (Aβ) peptides and in particular the 42-amino acid amyloid beta peptide (Aβ1-42). Aβ is an amphipathic peptide, the abundance of which is increased by gene mutations and risk factors linked to AD. Fibrils formed from Aβ constitute the cores of amyloid senile plaques, which are hallmarks of AD brain. Analogous fibrils generated in vitro are lethal to cultured brain neurons. These findings provided the central rationale for the original amyloid cascade hypothesis, a theory in which memory loss was proposed to be the consequence of neuron death caused by fibrillar Aβ (Hardy and Higgins (1992) Science 256:184-185).
Despite its strong experimental support and intuitive appeal, the original amyloid cascade hypothesis has proven inconsistent with key observations, including the poor correlation between dementia and amyloid senile plaque burden (Katzman (1988) Ann. Neurol. 23(2):138-144). Using a transgenic mouse model of AD, two surprising findings were obtained when the mice were treated with monoclonal antibodies against Aβ: (1) vaccinated mice showed reversal of memory loss, with recovery evident in 24 hours; and (2) cognitive benefits of vaccination accrued despite no change in senile plaque levels (Dodart et al. (2002) Nat. Neurosci 5:452-457; Kotilinek et al. (2002) J. Neurosci. 22:6331-6335). Such findings are not consistent with a mechanism for memory loss dependent on neuron death caused by amyloid fibrils.
Salient flaws in the original amyloid cascade hypothesis have been eliminated by an updated amyloid cascade hypothesis that incorporates a role for additional neurologically active molecules formed by Aβ self-assembly. These molecules are amyloid β-derived diffusible ligands (ADDLs), which assemble from Aβ1-42 at low concentrations (Lambert et al. (1998) Proc. Natl. Acad. Sci. USA 95:6448-6453). Essentially the missing links in the original amyloid cascade hypothesis, ADDLs rapidly inhibit long term potentiation (Lambert et al. (1998) Proc. Natl. Acad. Sci. USA 95:6448-6453; Walsh et al. (2002) Nature 416: 535-539; Wang et al. (2002) Brain Res. 924:133-140), a classic experimental paradigm for memory and synaptic plasticity. In the updated Aβ cascade hypothesis memory loss stems from synapse failure, prior to neuron death, with failure being caused by ADDLs, not fibrils (Hardy and Selkoe (2002) Science 297:353-356). ADDLs occur in brain tissue and are strikingly elevated in AD brain tissue compared to age matched controls (Kayed et al. (2002) Science 300:486-489; Gong et al. (2003) Proc. Natl. Acad. Sci. USA 100:10417-10422) and in AD transgenic mice models (Kotilinek et al. (2002) J. Neurosci. 22:6331-6335; Chang et al. (2003) J. Mol. Neurosci. 20:305-313).
A simplistic mechanistic approach to this theory can be illustrated as follows:

where formation of ADDLs is a separate pathway from formation of amyloid senile plaque both of which are in equilibrium with monomeric Aβ1-42.
Further experiments have shown important neurological properties of ADDLs. ADDLs were shown to have selective toxicity to hippocampal CA1 neurons compared with CA3 neurons, and the complete absence of toxicity towards cerebellar neurons (Kim et al. (2003) FASEB J. 17:118-120). Ventricular injection of Aβ1-42 oligomers into wild-type rats resulted in rapid, compromised behavioral models with complete recovery occurring within 24 hours (Cleary et al. (2005) Nat. Neurosci. 8:79-84) and these deficits are attributed to higher order oligomers, specifically 12-mer oligomers (Lesne et al. (2006) Nature 440:352-357). ADDL binding to neurons occurs with high specificity and is localized to post-synaptic receptors on a subset of hippocampal neurons (Lacor et al. (2004) J. Neurosci. 24:10191-10200). This triggers the rapid and persistent up-regulation of the immediate early gene product arc, translation of which is activity dependent at polyribosomes localized to subsets of dendritic spines (Steward et al. (1998) Neuron 21:741-751; Guzowski et al. (2000) J. Neurosci. 20:3993-4001). More recently, ADDLs have been implicated as upstream activators of tau phosphorylation and have been shown to interfere with animal behavior at femtomolar levels (Matsubara et al. (2004) Neurobiol. Aging 25:833-841).
The reversibility of memory loss in mouse models, coupled with the neurological properties of ADDLs and their presence in an AD brain, provides strong support for the hypothesis that AD is a disease of ADDL-induced synaptic failure (Lambert et al. (1998) Proc. Natl. Acad. Sci. USA 95:6448-6453; Klein et al. (2001) Trends Neuroscis. 24:219-220; Selkoe (2002) Science 298:789-791).
The use of antibodies specific to ADDLs is a powerful way to modulate the equilibrium between monomeric Aβ1-42 and ADDLs thereby providing treatment for disease conditions mediated by ADDLs. However, antibody delivery is typically limited to injectable solutions which pose patient compliance issues as well as the presence of an attending clinician. Small molecules that modulate this equilibrium, deliverable by non-injectable means such as oral delivery, transdermal delivery, pulmonary delivery, nasal delivery, etc. would be particularly beneficial.
A number of small molecules developed originally as amyloid fibril blockers are purported to possess Aβ oligomer assembly blocking properties. Some of these compounds include Alzhemd™ (Gervais (2004) Neuirobiol. Aging 25:S11-12), Clioquinol (Ritchie et al. (2003) Arch. Neurol. 60:1685-1691), substituted β-cyclodextrins (Yu et al. (2002) J. Mol. Neurosci. 19:51-55), trehalose (Lui (2005) Neurobiol. Disease 20:74-81), simple amino, carbonyl, and nitro substituted phenols (De Felice et al. (2001) FASEB J. March 20; De Felice et al. (2004) FASEB J. 18:1366-1372), Curcumin (Yang et al. (2005) J. Biol. Chem. 280(7):5892-5901), cyclohexanehexol analogs (McLaurin et al. (2006) Nature Med. 12:801-808), spirosterols (Lecanu et al. (2004) Steroids 69:1-16) and tricyclic pyrones (Maeqawa et al. (2006) J. Neurochem. 98:57-67). Two of these compounds, Alzhemd™ and Clioquinol, have progressed into clinical trials.
Alzhemed™ (3-amino-1-propanesulfonic acid), a so-called “GAG mimetic,” is proposed to reduce soluble and insoluble amyloid levels by binding to Aβ monomer, although no experimental details have appeared to confirm the proposed mode of action. Alzhemed™ has recently completed a 20 month open-label extension of a Phase II trial, and there are reports of slowed cognitive decline in some patients with mild AD, however, no efficacy was observed during the blinded phase of the study (Gervais (2004) Neuirobiol. Aging 25:S11-12).
The second compound in a phase II clinical trial, Clioquinol, was shown to stabilize the patients' cognitive ability compared to untreated patients and showed lower Aβ1-42 levels in their plasma (Ritchie et al. (2003) Arch. Neurol. 60:1685-1691). However, a toxic impurity (a di-iodo form of Clioquinol) made during production has resulted in the study being halted and Clioquinol being replaced with an analog termed PBT2 (Blennow et al. (2006) Lancet 368:387-403).
Lastly, an unidentified compound or compounds from an extract of ginko biloba leaves was reported to lower the levels of Aβ1-42 trimers and tetramers and increase the levels of high molecular weight polymers in a dose dependent manner (Yao et al. (2001) Brain Res. 889:181-190). Dose dependent protection against Aβ oligomer induced toxicity to PC-12 cells was also reported.
Of the compounds reported to block Aβ assembly or bind to Aβ1-42 monomer, few appear to have high therapeutic potential. Given its very simple structure and hydrophilic properties, it is highly unlikely that Alzhemed™ has high and selective affinity for Aβ1-42 monomer. Any effect that Alzhemed™ has on Aβ aggregation or disaggregation is likely attributable its interaction with ionic residues near the N-terminus of Aβ1-42. The cyclodextrins do not have either lead-like or drug-like properties that would recommend them for development (Oprea et al. (2001) J. Chem. Inf. Comput. Sci. 41:1308-1315; Vieth et al. (2004) J. Med. Chem. 47:224-232), and the phenols of De Felice contain aldehyde and nitro functionalities that are often considered reactive and excluded from screening libraries (Walters and Namchuk (2003) Nat. Rev. 2:259-266). A number of molecules containing the phenol functionality have been reported as “frequent hitters” in screening libraries (Roche et al. (2002) J. Med. Chem. 45:137-142). Thus, further evaluation of the activity and selectivity of the phenols of De Felice is needed to confirm that these compounds are valid hits. Some compounds with a steroidal backbone have been reported to be promiscuous inhibitors due to an unexpected self aggregation process (McGovern et al. (2002) J. Med. Chem. 45:1712-1722), which may explain the ambiguous spirosterol results. Finally, the active ingredient in the ginko biloba extract is unknown. Thus, most of the purported Aβ assembly blockers would not be considered compounds for therapeutic development.
Notwithstanding these putative results and as noted above, binding assays indicate that these compounds are, at best, moderate antagonists to ADDL formation.
Accordingly, it would be particularly beneficial to provide for small molecules which provide enhanced inhibition, regulation, and/or modulation of ADDL formation to improve cognitive function.